U.S. Survey Feet Versus International Feet
On a recent webinar that discussed the use of point clouds and modeling, I was impressed with my colleagues and the continuing steps they take to educate the profession about the process of accurate building documentation. However, one item which concerned me was the topic of working on large, campus-style projects.
In the presentation, it was pointed out that the georeferenced point cloud did not match the constructed model, although both were supposedly working with the same coordinate system. It was decided that the best way to resolve the issue was to move the point cloud to the model, after it was repeatedly said that users may never move the point cloud. This highlights an important issue which is not being discussed within the industry: What are the working units of the programs we are all using?
Consultants in the 3D industry who are not trained or are unfamiliar with surveying or geomatics systems may be surprised to learn that there are two different measurements for the length of a foot in the United States: the International Foot (also commonly called the foot) and the U.S. Survey Foot. The International Foot (which we were all taught in school) is defined as 0.3048 meters, whereas the U.S. Survey Foot is defined as 0.3048006096 meters. The difference of the two equates to 2 parts per million.
For example, in a measurement of 10,000 feet, the difference would be 0.02 feet (just less than one-quarter of an inch). In a measurement of 1 million feet, the difference is 2 feet.
While this difference is immeasurable to most, especially when dealing with short distances, when working with geospatial data, the difference is noticeable and troubling, specifically if the program you are utilizing does not recognize the difference between the two units. It can impact line work, coordinate readouts and modeled elements.
This difference comes into play, especially when you are using multiple design or as-built programs and you are unfamiliar with the working units or the topic of International Feet versus U.S. Survey Feet. This has caused numerous explanations, reviews and project overages for myself and my colleagues. While trying to resolve this issue, I hope to shed some light onto the programs that we are using and their expertise or limitations in this geospatially coordinated world we live in.
While I am not versed in every program out there, I do know a few very well and will highlight how each of those handles U.S. Survey Feet. It is important to remember that International Feet and U.S. Survey Feet are two separate units; to get them to integrate accurately, a conversion factor must be applied. Additionally most programs work in metric units behind the scenes and are scaling all visible objects (line work, coordinates, modeled features) to your working units.
Autodesk products (the majority of them) utilize International Feet (“Feet – Imperial” in the help window), with no option for U.S. Survey Feet. From the research I have done, AutoCAD Civil3D and ReCap (the new Autodesk product for point clouds) are the only Autodesk programs that allow for U.S. Survey Feet. Autodesk has “a project in the works to align the coordinates systems across our products, and the output of that project may be support for U.S. Survey Feet,” the company said.
Leica Geosystems produces a variety of programs and plugins under the Cyclone name. All Leica Cyclone products support both International Feet and U.S. Survey Feet. More importantly, Leica Cyclone produces plugins to load point cloud data into AutoCAD and REVIT, called CloudWorx for AutoCAD and CloudWorx for REVIT, respectively. Provided that the data is registered and georeferenced in U.S. Survey Feet, Leica’s plugins will load the point cloud into its correct geospatial location. From discussions with Leica representatives and personal experience, Leica CloudWorx produces a true geospatial solution in Autodesk products, producing the actual geospatial location, even if the working units are International Feet, by understanding the working units of the program and the file that the point cloud is loaded into.
Bentley Systems' MicroStation V8i supports U.S. Survey Feet as one of its default units. Previous versions did not support U.S. Survey Feet as a default, although MicroStation supplied instructions for users to modify the units’ .def files to allow the units to be set as default.
The first step in this process is to determine the units that your project needs to be delivered in and if the programs utilize that unit. Next, determine if the geospatial relationships of the deliverables are required and then what units the supplied control coordinates are in.
In the event you are using a package that does not support the units you just listed, you should get the project team together and discuss the issue and to determine the most practical workflows. Most critical is that the project team includes a technical person familiar with both the programs you are utilizing and the geospatial relationships of the data. Once a workflow is developed, publish it and make sure the entire project team has the understanding of the workflows to produce the deliverables correctly.
If you are using a package that does not support U.S. Survey Feet, there are a few workflows that are commonly accepted; however, it is important that before doing any of these workarounds you have discussions with team and a technical person familiar with both the program you are utilizing and geospatial relationships.
In the end, you have three choices:
- Do nothing: Create the file as you normally would and ignore the fact that you are utilizing different units. This may not produce the true geospatial solution, but it is done.
- Moving / shifting: Provided that your file is not depicting anything that is nearly 10,000 feet in length, another acceptable workflow is to move the file (preferably the model and not the point cloud) to the true geospatial location.
- Coordinate adjustment / scaling: When the desired product is U.S. Survey Feet, any coordinates input where the units are in International Feet (this includes the majority of Autodesk products) should be scaled up from 0,0,0 by a factor of 1.000002. This means that if a project is in U.S. Survey Feet, you must first calculate its respective coordinates at the International Feet values and then input those as the controlling coordinates. This will maintain the spatial relationship of the files. This is similar to a process where a member of the design team supplied a file in meters when U.S. Survey Feet is the desired deliverable.
In a subsequent article, I hope to write about other packages that can and cannot utilize U.S. Survey Feet and hope to hear from a few of you also about your struggles. As we move more into a geospatial solution and our peers and colleagues are educated on this topic, we can move onto more pressing matters.